Where defendant was charged with resisting an executive officer, trial court erred by excluding defense expert testimony on the reasonable use of force by police, but the error was harmless. After an unsuccessful attempt to evade a traffic stop, Reardon, who was under the influence of drugs and alcohol, did not comply with police officers’ instructions and started acting erratically. An officer tackled Reardon but was unable to control him. A physical altercation ensued where officers used “distraction blows” to Reardon’s head and struck him multiple times with a baton in an attempt at “pain compliance.” The officers and civilian witnesses gave various accounts of the incident. At trial, Reardon sought to introduce expert testimony concerning the use of force by police. The court ordered the evidence excluded. Reardon was convicted of resisting an executive officer (Pen. Code, § 69) and other offenses. He appealed. Held: Affirmed on this point. To obtain an acquittal on a resisting charge, a defendant need only raise a reasonable doubt whether the officers’ actions were lawful. Although expert testimony is not always needed to show excessive force, there may be cases that involve police procedures and practices that are sufficiently beyond common experience so that an expert may be necessary. The defense expert here was prepared to testify about the proper use of batons and the proper use of escalating force on a noncompliant person. This testimony would have been appropriate and should have been admitted. The trial court was also wrong to exclude the evidence because it involved “an ultimate issue.” The court distinguished People v. Brown (2016) 245 Cal.App.4th 140, which held it was error to admit prosecution expert testimony that the officers in that case adhered to their training. However, Reardon failed to show he was prejudiced by the error. Evidence of Reardon’s earlier physical resistance before any possible excessive force was used supported the verdict.